Voltage-gated Na channels in many neurons, including several types in the cerebellum and brainstem, are specialized to allow rapid firing of action potentials. This rapid firing is due in part to resurgent Na current, an unusual current that flows upon repolarization from positive to negative potentials. Resurgent current flows through voltage-gated Na channel 1 subunits that are recovering through open states from block by an endogenous protein. Disruption of resurgent Na current correlates with reduced firing and motor deficiencies, including ataxia and tremor, whereas Na channel mutations implicated in pain and movement disorders increase both resurgent current and firing rates. Together, these findings suggest resurgent Na current is a key determinant of neuronal output and thus, normal motor function. Previous results indicate a role for the Na channel 2 subunit Nav24 as a blocker; however, Nav24 is not sufficient to produce resurgent Na current in all cell types. In the experiments proposed here, we will further investigate the biophysical and molecular determinants of block. We will evaluate structural requirements for block by testing the ability of a series of 24- derived peptides to block channels. We will also test the properties of 1 subunits that are necessary to permit resurgent current in an expression system. Confirming the identity of potential blocking proteins and identifying the biophysical mechanisms of block will help us to further understand the connection between resurgent Na current and rapid firing that is important for control of motor behaviors.